1. Field of the Invention
The invention relates to a method for operating a photovoltaic system, comprising solar panels, at least one DC/DC converter and an intermediate DC circuit.
2. Description of the Related Art
Photovoltaic systems serve to convert solar radiation into electrical energy. Such systems comprise individual solar cells, which are combined to form what are known as solar panels or photovoltaic panels. Depending on the size and type of the system, these are in turn electrically connected in series to form what are known as strings.
This adds together the voltage of the individual panels. If necessary, several of these strings can also be electrically connected in parallel with one another.
If this is a grid-tied photovoltaic system, the direct current generated in the solar panels is converted into alternating current by inverters so as to feed the generated electrical energy into the alternating current network, such as the public mains supply.
A maximum yield of electrical energy is achieved if the solar panels are operated in an operating point in which they output the maximum electrical power.
This optimal operating point MPP (Maximum Power Point) is achieved by controlling the load of the solar panels using an MPP tracking method.
In order to provide a sufficiently large variation range for the MPP tracking and to also be able to configure solar panels for a lower voltage, provision can be made for a multi-stage conversion of the voltage supplied by the solar panels. To this end, DC-DC converters are arranged upstream of the inverters. Their voltage transmission ratio is potentially highly variable, as a result of which the operating point of solar panels connected to the DC-DC converters can be varied over a wide range. Therefore with DC-DC converters that are formed as step-up converters, it is also still possible to feed into the power supply network if the output voltage of the solar panels is lower than the minimum required voltage of the inverter.
In particular, for cost reasons, the DC-DC converters are only designed for a limited input voltage range about the optimal operating point MPP (Maximum Power Point).
Such DC-DC converters are therefore not designed for the no-load voltage of the solar panels. This no-load voltage occurs, for instance, during sunrise or after a temporary shading of the solar panels, if no power is drawn from the solar cells.
In order, in these cases, to prevent damage to the DC-DC converters due to overvoltage, the solar panels are usually loaded during these phases with suitable facilities, such as contactors and resistors that are connected in parallel.